Cellular communications systems are commonly employed to provide voice and data communications to a plurality of mobile units or subscribers. Analog cellular systems, such as those designated AMPS, ETACS, NMT-450, and NMT-900, have been deployed successfully throughout the world. More recently, digital cellular systems such as those designated IS-54B in North America and the Pan-European GSM system have been introduced and deployed. These systems and other systems are described, for example, in the book entitled Cellular Radio Systems by Balston, et al., published by Artech House, Norwood, ME, 993.
Frequency reuse is commonly employed in cellular technology wherein groups of frequencies are allocated for use in regions of limited geographic coverage known as cells. Cells containing equivalent groups of frequencies are geographically separated to allow mobile units in different cells to simultaneously use the same frequency without interfering with each other. By doing so, many thousands of subscribers may be served by a system with only several hundred frequencies. In the United States, for example, Federal authorities have allocated to cellular communications a block of the UHF frequency spectrum that is further subdivided into pairs of narrow frequency bands called channels. Channel pairing results from the frequency duplex arrangement wherein the transmit and receive frequencies in each pair are offset by 45 megahertz (MHz). At present, there are 832, 30-KHz wide, radio channels allocated to cellular mobile communications in the United States. To address the capacity limitations of this analog system, a digital transmission standard designated IS-54B has been provided, wherein those frequency channels are further subdivided into three time slots.
In addition, capacity limitations have been addressed by using microcells, that is, low power cellular transmissions that provide coverage over a smaller area. The smaller microcelis allow more cells to exist within a predefined geographic area, thereby increasing the number of users that can be serviced within that geographic area. A particular application of microcell technology is indoor cellular radiotelephone services.
As illustrated in FIG. 1, an indoor cellular communication system 20 as in the prior art includes one or more mobile stations or units 22, one or more wall mounted base stations 24, a radio control interface 26, and a mobile switching center (MSC) 28. Although only one cell 30 is shown in FIG. 1, a typical indoor cellular network may have several cells 30, each cell usually being serviced by one or more wall mounted base stations 24. The number of wall mounted base stations 24 depends on the channel capacity of the cell 30. Each wall mounted base station typically supports anywhere from 4-12 channels, depending upon its site. The cell 30 typically has one or more control channels and one or more voice/data (hereafter referred to as "traffic") channels allocated to it. The control channel typically is a dedicated channel used for transmitting cell identification and paging information.
Each wall mounted base station 24 is connected to the radio control interface 26 by a radio interface link 32. The radio control interface 26 exchanges signals between the wall mounted base stations 24 and the mobile switching center 28. Specifically, the radio control interface 26 converts the traffic and control information from the format received over the radio interface links 32 into a format suitable for transmission over a dedicated transmission link 34 interconnecting the radio control interface (RCI) 26 to the MSC 28. In the reverse direction, the RCI 26 converts the traffic and control information received over transmission link 34 into a format suitable for transmission over radio interface links 32 to the respective base stations 24.
The MSC 28 is the central coordinating element of the overall cellular network 20. It typically includes a cellular processor 36 and a cellular switch 38, and provides an interface to the public switched telephone network (PSTN) 40. Through the cellular network 20, a duplex radio communication link 42 may be effected between two mobile units 22 and a landline telephone user 44. The function of the base stations 24 is commonly to handle the radio communications with the mobile units 22. In this capacity, the base stations 24 also supervise the quality of the link 42 and monitor the received signal strength from the mobile units 22.
A typical wall mounted base station 24 as in the prior art is schematically illustrated in FIG. 2. The base station 24 includes a radio link interface 50, a power supply 52, and one or more communication channel transceiver boards 54. In addition, the base station includes an antenna 56 associated with each of the communication channel transceiver boards 54. The radio link interface 50 provides an interface between the radio control interface 26 and the communication channel transceiver boards 54. In essence, the radio link interface 50 multiplexes/demultiplexes the signals to/from the radio control interface 26 for use by the communication channel transceiver boards 54. The power supply 52 provides power to the other components of the base station 24.
Further, the base station 24 includes a number N of communication channel transceiver boards 54 for effectuating radio communications with mobile units 22. Power for each of the communication channel transceiver boards is supplied by the power supply 52. Traffic and control signals are exchanged between the communication channel transceiver boards 54 and the radio link interface 50 over respective lines 58. In addition, the communication channel transceiver boards 54 are interconnected with one another by links 60 so that the transceiver boards 54 can receive traffic signals from more than one antenna 56 for reception diversity.
Each communication channel transceiver board 54 typically comprises a circuit board 66 having a single transceiver 68, for example, a radiotelephone, as illustrated in FIG. 3. The transceiver 68 includes a controller 70, a receive local oscillator (RXLO) 72, a transmit local oscillator (TXLO) 74, a frequency generator (FG) 76, a duplexer 78, an antenna 79, mixers 80 in the receive signal path, and mixers 81 in the transmit signal path. The controller 70 communicates with the RXLO 72, TXLO 74, and FG 76 to control the conversion of received radio frequency (RF) signals down to a baseband frequency for processing, and for the conversion of transmit signals to a radio frequency (RF) signal for transmission over the communication link 42. Further, the controller 70 exchanges traffic and control signals with the mobile switching center 28 via the radio link interface 50 and the radio control interface 26.
The RXLO 72, TXLO 74 and FG 76 are each configured to receive tuning information such as a division ratio from the controller 70 for generating a stable frequency signal used in performing conversion of the traffic signal in the receive signal path via mixer 80 and in the transmit signal path via mixer 81.
For example, in the receive signal path, the RF signal may be converted down to a receive intermediate frequency by subtracting the FG 76 signal from the RF signal at the mixer 80 associated with the FG 76. In general, the two signals are added by the mixer 80 and filtering is used to 30 isolate the resulting term that represents the difference of the two signals. The division ratio or tuning information is sent by the controller 70 to the FG 76 at power up or activation of the transceiver 68. This first conversion of of the received signal is referred to as the first down conversion stage in the received signal path. Next, the RXLO 72 further converts the received signal from the receive intermediate frequency to a base band frequency by further mixing the signal with a second frequency signal generated by the RXLO 72 at the mixer 80 associated with the RXLO 72. Filtering is then used to isolate the term that represents the difference of the two signals. The division ratio for the second frequency signal is also sent by the controller 70 to the RXLO 72 at power up. This is referred to as the second down conversion stage in the received signal path. At the base band frequency, the received traffic signal can then be processed by the digital control logic of the transceiver 68. In a like manner, traffic signals in the transmit signal path are initially converted from a baseband frequency to a transmit intermediate frequency in the first up conversion stage at the mixer 81 associated with the TXLO 74 using a division ratio provided to the TXLO 74 by the controller 70. The traffic signal is then converted up from the transmit intermediate frequency to a radio frequency in the second up conversion stage at the mixer 81 associated with a FG 76.
Note that the transmit intermediate frequency generally is mixed with the same local oscillator frequency used in the first down conversion in the received signal path. Therefore, if the same local oscillator frequency is used in both the first down conversion stage in the received signal path and the second up conversion stage in the transmit signal path, then only one FG 76 would be needed for transceiver 68. However, since the transmit and receive intermediate frequencies differ, a separate local oscillator may be necessary for each of the receive signal path and the transmit signal path so that the frequencies may be converted to/from the baseband frequency.
The RXLO 72, TXLO 74, and FG 76 can be programmed with respective division ratios in the following manner. During each power up or activation of the transceiver 68, the controller 70 sends several commands over a serial bus 84 which interconnects the controller 70 with the RXLO 72, the TXLO 74, and the FC 76. Each command sent over the serial bus 84 is addressed to a separate one of the RXLO 72, TXLO 74 and FG 76. The commands typically comprise a clock signal sent over one line of the bus, a M-bit data word sent over another line of the bus, and a select signal sent over one of the remaining lines of the serial bus, wherein each of the remaining bus lines is dedicated to one of the RXLO 72, TXLO 74 or FG 76. Each of the RXLO 72, TXLO 74 and FG 76 has a register that receives the data word sent over the serial bus, though the data is only read into the device if a select signal designating that device is also received. Otherwise, the data word is merely shifted out of the register and the data word associated with the next select signal is shifted in with the next command. Thus, when the controller 70 writes to the RXLO 72, all three devices receive the data word but only the RXLO 72 actually reads because it is the only device that receives a select signal that identifies the RXLO 72.
Indoor cellular systems such as the one described above have become widely popular for several reasons. First, a cellular customer can use the same telephone everywhere he/she goes. Second, the cellular telephone of the customer does not need additional circuitry to allow for indoor use as do some cellular telephones that operate as cordless telephones indoors. Third, indoor cellular systems provide larger coverage areas indoors than do typical cellular telephones operating in a cordless mode indoors. Fourth, the capacity of an indoor cell can be increased easily by adding additional wall mounted base stations.
However, a limitation of indoor cellular systems is the size and cost of the wall mounted base stations 24. The size of a wall mounted base station 24 can easily become obtrusive when designed to include more circuit boards 66 so as to increase channel capacity. Thus, endlessly adding more circuit boards to increase the number of channels is usually not a viable option. Therefore, additional wall mounted base stations must be added at a sizeable cost. Therefore, a need exists in the market for smaller and less obtrusive wall mounted base stations that cost less and service more channels.